Re: [PATCH 07/10] module: __rcu annotations

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Feb 25, 2010 at 12:07:47AM +0100, Arnd Bergmann wrote:
> On Wednesday 24 February 2010 23:17:49 Paul E. McKenney wrote:
> > > Ok, this is a significant limitation of the list rcu annotation
> > > then, it's not possible to pass the same list into list_for_each_entry
> > > and list_for_each_entry_rcu with the way I changed the rcu list
> > > definition. I would be possible to do a __list_for_each_entry_rcu
> > > macro that takes an rcu_list_head but does not actually use
> > > rcu_dereference, but I'm not sure if that's good enough.
> > 
> > Hmmm...  If the __rcu annotation was visible at runtime, it would be
> > easy provide an annotated version of list_for_each_entry_rcu() that
> > checks for module_mutex being held under lockdep.
> 
> Well, if we keep the struct rcu_list_head and make it mandatory for
> rcu protected lists, it could be defined as
> 
> struct rcu_list_head {
> 	struct list_head head;
> #ifdef CONFIG_PROVE_RCU
> 	bool (*check)(void);
> #endif
> };
> 
> #ifdef CONFIG_PROVE_RCU
> #define RCU_LIST_HEAD_INIT_CHECK(__head, __check) \
> 	{ .head = LIST_HEAD_INIT((__head).head), .check = (__check) }
> #else
> #define RCU_LIST_HEAD_INIT_CHECK(__list,__check) {.head = LIST_HEAD_INIT((__head).head) }
> #endif
> 
> #define RCU_LIST_HEAD_INIT(head) RCU_LIST_HEAD_INIT_CHECK(head,&rcu_read_lock_held)
> #define RCU_LIST_HEAD_INIT_BH(head) RCU_LIST_HEAD_INIT_CHECK(head,&rcu_read_lock_bh_held)
> 
> #define list_entry_rcu_check(ptr, type, member, check) \
>         container_of(rcu_dereference_check((void __rcu __force *)(ptr), check), type, member)
> 
> #define list_for_each_entry_rcu(pos, __head, member) \
>         for (pos = list_entry_rcu_check((__head)->head.next, typeof(*pos), \
> 					 member, (__head)->check); \
>                 prefetch(pos->member.next), &pos->member != (head); \
>                 pos = list_entry_rcu_check(pos->member.next, typeof(*pos), \
> 					 member, (__head)->check)))
> 
> That would let us check all the heads for correct usage, and at the same
> time avoid having to annotate all the list entries.

Cool!!!

The nice thing about this is that we don't end up with the API explosion
for the RCU list primitives.  However, it does require that a given
rcu_list_head have a single synchronization-design rule for all uses.
Of course, if there were multiple rules, one could construct a check
that was simply the union of all the rules, but that would miss some
types of errors.

Of course, if this became a problem, there could be an argument to the
->check function that the normal list_for_each_entry_rcu() defaults to
"no effect".

Or is there a better way to handle this?

							Thanx, Paul